Re: Not quite managing my first join - Mailing list pgsql-novice
From | Bob Henkel |
---|---|
Subject | Re: Not quite managing my first join |
Date | |
Msg-id | 762e5c0504091754c4a87b7@mail.gmail.com Whole thread Raw |
In response to | Not quite managing my first join ("Rodolfo J. Paiz" <rpaiz@simpaticus.com>) |
List | pgsql-novice |
On Apr 9, 2005 9:39 PM, Todd Lewis <lewis-todd@sbcglobal.net> wrote:
Try this->http://www.postgresql.org/docs/8.0/interactive/functions-conditional.htmlIs there anything in postgre like the NVL() function in Oracle?
Bob Henkel wrote:
>
>
> On Apr 9, 2005 4:47 PM, *Bob Henkel* <luckyratfoot@gmail.com
> <mailto:luckyratfoot@gmail.com>> wrote:
>
>
>
> On Apr 9, 2005 3:49 PM, *Rodolfo J. Paiz* <rpaiz@simpaticus.com
> <mailto:rpaiz@simpaticus.com>> wrote:
>
> Hi!
>
> I'm within a hair of getting my first join to work, but
> something is
> wrong and for the life of me I can't figure out why. Hints,
> anyone?
>
> ====================================================================
> select t1.month,t1.single,t2.multi from (
> select to_char(date, 'YYYY-MM') as month, sum(hrs_total) as
> single
> from flights,aircraft
> where flights.callsign=aircraft.callsign and
> aircraft.engines=1
> group by month order by month) as t1
> full outer join (
> select to_char(date, 'YYYY-MM') as month, sum(hrs_total) as
> multi
> from flights,aircraft
> where flights.callsign=aircraft.callsign and
> aircraft.engines=2
> group by month order by month) as t2
> on (t1.month = t2.month);
>
> month | single | multi
> ---------+--------+-------
> 2003-08 | 5.6 |
> 2003-09 | 3.5 |
> 2003-10 | 4.2 |
> 2003-11 | 17.7 |
> 2003-12 | 16.4 |
> 2004-01 | 13.3 |
> 2004-02 | 1.7 | 7.6
> 2004-03 | 12.2 |
> 2004-04 | 13.3 | 4.1
> 2004-05 | 7.3 |
> 2004-06 | 7.1 | 9.4
> 2004-07 | 2.8 | 7.6
> 2004-08 | 6.0 | 7.0
> 2004-09 | 7.9 | 2.4
> 2004-10 | 2.5 |
> | | 17.4
> 2004-12 | 5.8 | 3.1
> | | 10.1
> | | 14.1
> 2005-03 | 9.5 | 2.7
> (20 rows)
> ====================================================================
>
> The problem is that the query seems to be returning all the
> correct
> results, *BUT* I have two problems:
>
> - Instead of zero where I did not fly a multi-engine airplane, it
> shows no value at all.
>
> - For columns where t1 had no value, no month is shown.
>
> How can I fix those two little details? My brain is
> temporarily fried
> and I'm not understanding the docs very clearly.
>
> Thanks!
>
> --
> Rodolfo J. Paiz <rpaiz@simpaticus.com
> <mailto:rpaiz@simpaticus.com>>
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 6: Have you searched our list archives?
>
> http://archives.postgresql.org
>
> Well it looks like your outer join is doing exactly like it should.
> >From the docs at
> http://www.postgresql.org/docs/8.0/interactive/queries-table-expressions.html
>
> "FULL OUTER JOIN
>
> First, an inner join is performed. Then, for each row in T1
> that does not satisfy the join condition with any row in T2, a
> joined row is added with null values in columns of T2. Also,
> for each row of T2 that does not satisfy the join condition
> with any row in T1, a joined row with null values in the
> columns of T1 is added."
>
> Here is a simple example that shows your problem and how you can
> turn those nulls into something
>
> CREATE TABLE child
> (
> child_id int8,
> child_name varchar(50)
> ) ;
>
> CREATE TABLE parent
> (
> parent_id int8,
> parent_name varchar(50),
> child_id int8
> ) ;
>
>
> insert into parent values (1,'Mommy Kim',89);--HAS A CHILD
> insert into parent values (1,'Mommy Lisa',NULL);--HAS NO CHILD
> insert into child values (89,'Child Chucky');--HAS A MOTHER
> insert into child values (109,'Child Bruce');--HAS NO MOTHER
>
>
> SELECT p.parent_name,c.child_name
> FROM public.parent as p
> FULL JOIN public.child as c ON p.child_id = c.child_id;
>
> SELECT CASE WHEN p.parent_name IS NULL
> THEN 'HAS NO CHILD' ELSE p.parent_name
> END,
> CASE WHEN c.child_name IS NULL
> THEN 'HAS NO PARENT' ELSE c.child_name
> END
> FROM public.parent as p
> FULL JOIN public.child as c ON p.child_id = c.child_id;
>
> Not that this is a big deal, but the has no parent and has no child
> strings should be switched so it reads correctly.
9.13.2. COALESCE
COALESCE
(value [, ...])
The COALESCE
function returns the first of its arguments that is not null. Null is returned only if all arguments are null. This is often useful to substitute a default value for null values when data is retrieved for display, for example:
SELECT COALESCE(description, short_description, '(none)') ...
Like a CASE expression, COALESCE
will not evaluate arguments that are not needed to determine the result; that is, arguments to the right of the first non-null argument are not evaluated.
9.13.3.
pgsql-novice by date: